Galvanized aluminum sheet

ABSTRACT

A method for applying a zinc layer onto an aluminum or aluminum alloy sheet, comprising pretreating the surface and applying the layer by electrolytic galvanizing, the pretreating comprises electrochemical graining of the surface, for example in a solution having a pH less than 3 with an alternating current applied between the sheet and an electrode. In an alternative method, the pretreating includes applying a preliminary zinc layer by immersing the surface in a zinc-containing alkaline solution, applying a potential to the sheet and reversing the polarity of said potential at least once. The sheets are useful for building cladding sheets and automotive panels.

TECHNICAL FIELD

The invention relates to methods for applying a layer comprising zinconto at least one surface of an aluminum or aluminum alloy sheet,comprising in succession a pretreatment step and an electrolyticgalvanizing step. The invention also relates to the galvanized aluminumor aluminum alloy sheet produced by the methods, and the use of suchsheet in building structures and as automotive body sheet. In thefollowing description, for brevity we refer to aluminum sheets, but thisterm is to be understood to include aluminum alloy sheets.

BACKGROUND ART

Aluminum sheet is used on a wide scale in building structures asinterior and/or exterior panels of buildings both for facade and roofingstructures. An advantage of this is that the low specific weight of thealuminum means that the building structure may be made considerablylighter than for example with steel sheet. A disadvantage of untreatedaluminum sheet is that the building structure reflects a large amount oflight which limits its application in the immediate vicinity orairfields, for example. A solution to this problem is to provide thealuminum sheet with a surface layer, for example zinc, while retainingthe structural advantages. An advantage of a galvanized aluminum sheetis that it reflects less light and has good corrosion resistance. Afurther advantage of galvanized aluminum sheet is that the buildingstructure made with it requires little maintenance due to the durabilityof the AlZn system. A further advantage of galvanized aluminum sheet isthat the appearance of the aluminum sheet changes slowly over time, i.e.it "lives". This last property, the so-called patina effect, is muchsought after by architects for application in buildings of their design.Galvanizing the aluminum sheet considerably increases the applicabilityof the aluminum sheet.

Methods of electrolytically plating zinc onto aluminum (galvanizing) arewell known. To improve the applied zinc layer, pretreatments of thealuminum surface have been proposed.

JP-A-52005630 discloses electroplating one of Cu, Ni, Zn, Sn, Pb, Cd andCr onto the chemically roughened surface of an aluminum or aluminumworkpiece. The roughening is performed in two stages: first in alkalipH≧11 or a fluoric acid solution, and second using a mixed mineral acidsolution.

In EP-A-0497302 a pretreatment step consisting of degreasing andpickling is performed and cathodic zinc electroplating is then carriedout in two steps, thereby forming two adjacent zinc layers. The layersmay contain Ni or Fe additions to improve bonding.

Another method for pretreating and galvanizing an aluminum sheet isgiven in EP-A-0498436, which describes a method for the continuouselectrolytic application of a zinc layer onto an aluminum sheet intendedfor processing into an automotive body sheet, after which the aluminumsheet is provided with a paint layer. The method comprises in successionthe steps (i) alkaline degreasing, (ii) pickling in an acid solution,(iii) anodizing and cathodic zinc electrolysis of the aluminum sheet inthe same acid solution. In between the steps the aluminum sheet iscleaned by rinsing with water. The anodization and electrolysis processis not dependent on the extent of pretreatment, which may even beomitted. A disadvantage of galvanized aluminum sheet obtained by thismethod is that the bonding of the applied zinc layer on the aluminumsheet is very poor when the galvanized aluminum sheet is highlydeformed, for example by bending.

Zinc plating of wrought aluminum sheet as a precursor to zinc phosphatetreatment and painting is described in U.S. Pat. No. 5176963. The zincplating may comprise two stages, namely displacement plating from analkaline bath and electroplating from an acid bath. There is nodisclosure of the application of a potential to the sheet during thedisplacement plating stage.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a method for pretreating andgalvanizing an aluminum sheet by which a very effective bond is obtainedbetween the applied zinc layer and the aluminum sheet, the bondremaining effective under large subsequent deformation of the sheet. Anadditional object of the invention is that the method for pretreatingand galvanizing the aluminum sheet may be carried out as a continuousprocess.

According to the invention in one aspect there is provided a method forapplying a layer comprising zinc onto at least one surface of analuminum or aluminum alloy sheet, comprising the steps of pretreatingsaid surface and applying the layer by electrolytic galvanizing,characterized in that the pretreating step comprises electrochemicalgraining of said surface.

Electrolytic graining is a process known in the art and is one exampleof the technique of graining. It is known in the art (see "Study of themechanism of the A.C. electrolytic graining of aluminum" by P. Laevers,Brussels Free University, November 1995 and EP-A-586504) to useelectrolytic graining for roughening of aluminum sheet, typically analuminum-litho sheet, but the use of graining as a pretreatment inelectrolytic galvanizing of aluminum sheet is novel.

By the electrochemical graining pretreatment step of the invention aneffective bond between the electrolytically applied zinc layer and thealuminum sheet is formed, the bond remaining effective during subsequentdeformation of the aluminum sheet, for example by bending. It can alsoachieve the effect that the resulting aluminum sheet has a very goodcorrosion resistance. Furthermore the method may be carried out in acontinuous process.

The invention is based in part on the insight that to obtain awell-bonded zinc layer on the aluminum sheet so that the bond remainseffective under great deformation of the galvanized aluminum sheer, thepretreatment is extremely important. It is believed that theelectrochemical graining of the aluminum sheet produces a roughenedsurface so that the subsequently electrolytically applied zinc layer isalso mechanically bonded. This mechanical bonding is partly responsiblefor achieving the effect that the zinc layer remains bonded under largedeformation of the galvanized aluminum sheet.

Very good results, including effective bonding of the electrolyticallyapplied zinc layer may be obtained with graining process parameterscomprising one or more of:

(a) alternating current, preferably with a frequency in the range from10-1000 Hz, more preferably 40-100 Hz;

(b) current density of 5-200 A/dm², preferably 10-100 A/dm², and morepreferably 20-50 A/dm² ;

(c) bath temperature 15-60° C., preferably 40-50° C.;

(d) treatment time 0.2-60 s, preferably 0.5-10 s, and more preferably2-3 s;

(e) bath acidity pH<3, and preferably pH<2.

A further advantage is that these process parameters lend themselves toapplication in a continuous process operation.

As mentioned, the electrolytic graining is preferably performed in anacid solution. Various acids may be used for this, but preferably use ismade of a hydrochloric acid solution or a nitric acid solution. Thegraining step may be a part of a pretreatment cycle. A typical preferredpretreatment cycle comprises in succession:

(1) alkaline degreasing;

(2) electrolyte graining;

(3) anodizing e.g. in a sulphate solution; and may also comprise rinsingwith water between these steps. However, the method is not limited tothis pretreatment cycle. Better alkaline degreasing is achieved ifsimultaneously a direct current is applied in a range 1-20 A/dm²,preferably 5-15 A/dm².

Preferably anodizing is carried out after the electrolytic graining.

In a further aspect, the invention consists in an aluminum or aluminumalloy sheet having an electrolytically applied layer comprising zinc inwhich said layer is adjacent to an aluminum or aluminum alloy surfaceroughened by electrochemical graining. By microscopic inspection of thealuminum-zinc interface it will be possible to detect the electrolyticgraining.

In a second method, the invention provides a method for applying a layercomprising zinc onto at least one surface of an aluminum or aluminumalloy sheet, comprising the steps of pretreating said surface andapplying said layer by electrolytic galvanizing, said pretreating stepincluding applying a preliminary layer comprising zinc onto saidsurface, characterized in that the application of said preliminary layercomprises (i) immersing said surface in a zinc-containing alkalinesolution, (ii) applying a potential to the sheet to cause anelectrolytic current to flow, and (iii) reversing the polarity of saidpotential at least once. This method may be combined with theelectrolytic graining pretreatment described above.

By this method of applying a preliminary zinc layer, a very effectivebond of the subsequent electrolytically applied zinc layer may therebybe obtained. An advantage of this pretreatment is that the wholepretreatment may be carried out in an alkaline solution. Thus only asingle waste flow, which is alkaline, may be formed which represents alogistical advantage. In addition the aluminum sheet can be directlydegreased. The entire pretreatment cycle can therefore consistessentially of one pretreatment step.

Zincate treatments are known for applying conversion layers ontoaluminum, for example as known from "Oppervlaktebehandelingen vanaluminum" by T. van der Klis and J. W. du Mortier published by theVereniging voor Oppervlaktetechnieken voor Materialen, Bilthoven, NL,3rd edition 1992, pp 406-409. A basic composition for a zincate picklecomprises 40-50 g/l ZnO and 400-500 g/l NaOH. However, a zincatetreatment according to the present method, besides applying a thin zinclayer onto the aluminum sheet, also activates the surface of thealuminum sheet so that the zinc layer applied electrolyticallysubsequently bonds better. The thickness of the layer comprising zincproduced in the pretreatment step is not of major importance, but may bein the range 0.1 to 0.5 g/m², and the ultimate layer thickness of thezinc is essentially determined during the electrolytic galvanizingstage.

In this second method of the invention, the aluminum sheet may beanodized, e.g. in a sulphate solution, prior to the electrolyticgalvanizing. This can achieve the effect that the electrolyticallyapplied zinc layer bonds very effectively to the aluminum sheet.

In the second method of the invention, during the zincate treatment, thealuminum sheet changes polarity at least once, preferably at leasttwice. Thus the aluminum sheet may be initially anode-connected (A) fora given time, and then cathode-connected (C). This connection sequencemay be designated A-C. The aluminum sheet is preferably anode-connectedfirst so that a part of the oxide layer goes into solution, after whicha thin zinc layer deposits onto the aluminum sheet at the time when thealuminum sheet is cathode-connected. However C-A connection is alsopossible. The final connection is preferably as a cathode. Preferablythe aluminum sheet is connected at least A-C-A-C. The duration ofmaintenance of each polarity is here called the electrolysis time perpolarity, and is preferably at least 0.5 s.

Very good results may be obtained with zincate process parameterscomprising one or more of:

(a) bath temperature 10-60° C., preferably 20-30° C.;

(b) current density of 0.5-20 A/dm², preferably 2-10 A/dm² ;

(c) electrolysis time per polarity 0.5-10 s, preferably 1-4 s;

(d) total immersion time 1-30 s, preferably 2-10 s;

(e) bath composition comprising 10-300 g/l NaOH and 2-40 g/l ZnO,preferably 50-150 g/l NaOH and 5-20 g/l ZnO.

An advantage of these process parameters is that the pretreatmentprocess lends itself well to application in a continuous processoperation.

Following the pretreatment according to both methods of the inventionthe aluminum sheet is electrolytically galvanized in an acid solution.Conventional galvanizing techniques may be used, and many differentprocesses are suitable. Good results may be obtained when thegalvanizing process parameters comprise one or more of:

(a) direct current with a current density of 10-100 A/dm², preferably30-70 A/dm² ;

(b) bath temperature 20-70° C., preferably 40-60° C.;

(c) treatment time 5-90 s, preferably 10-40 s;

(d) electrolyte composition comprising zinc sulphate with 30-200 g/lzinc, preferably 60-120 g/l;

(e) acidity of the electrolyte pH<4, preferably approximately pH 2.5.

Using these parameters, a well-bonded layer comprising essentially zincmay be applied to the pretreated aluminum sheet, the bonding remainingeffective under a large deformation of the galvanized aluminum sheet. Afurther advantage is that it is possible to perform a continuousprocess. With a pH of approximately 2.5 it is possible to buffer theelectrolyte, thereby making the electrolytic galvanizing processconsiderably more stable.

The electrolyte composition used is not limited to a compositioncomprising a sulphate solution, and for example a chloride solution mayalso be applied.

According to a preferred version of the first method of the invention,wherein the pretreatment comprises electrolytic graining followed by abrief anodization after which the aluminum sheet is electrolyticallygalvanized, desirably the same electrolyte liquid bath is not used forthe anodizing as for the galvanizing. Working with a continuous processoperation, but one in which the electrolytes for the anodizing and thegalvanizing are separate, has the advantage that the two processconditions may be controlled independently. Thus for the anodizingprocess preferably a pH<2 is used, and for the electrolytic galvanizingpreferably a pH of approximately 2.5. Moreover, the electrolyte for theanodizing may comprise either zinc sulphate or an iron sulphate, andalso such metals as Ni or Cu, while the electrolyte for the galvanizingis preferably zinc sulphate.

Both methods in accordance with the invention are preferably carried outin a continuous process, although it is possible to carry them outbatchwise.

As mentioned, the methods in accordance with the invention are suitablefor galvanizing aluminum sheets made of aluminum and a wide range ofaluminum alloys, such as aluminum alloys of the 1xxx type, the 3xxxtype, and the 6xxx type, but also of the 2xxx type and the 5xxx type (AAdesignations).

The zinc layer applied in the methods of the invention may beessentially a pure zinc layer or may be primarily zinc but includingminor amounts of impurity elements or deliberately added elements, as isknown in the art. Typically such impurity elements or added elements arepresent at less than 10%, more usually less than 5% by weight in thezinc layer.

Another advantage of the methods in accordance with the invention isthat the galvanized aluminum sheet which is produced has an atmosphericcorrosion resistance almost comparable to that of zinc sheet, which hasbeen used for application in building structures. This enablesstructures to be made lighter while retaining the good corrosionresistance. Moreover, the total zinc consumption when using galvanizedaluminum sheet in building structures is considerably less than whenusing zinc sheets.

The invention also consists in the galvanized aluminum sheet produced bythe methods according to the invention.

In a further aspect galvanized aluminum sheet, obtained by eithermethod, may be used in building systems such as facade and roofingstructures, and windowsill structures. Galvanized aluminum sheetsuitable for application in building systems such as those known underthe trade names KAL-ZIP and KAL-BAU may be obtained by the invention. Inthe case of application of the galvanized aluminum sheet in buildingstructures, the aluminum sheet preferably comprises a weight per unitarea of applied zinc in the range of 10-300 g/m², and more preferably30-100 g/m².

In another aspect galvanized aluminum sheet, obtained by either method,may be used in shaping applications such as for the manufacture ofautomotive body parts by pressing. In the case of use of the galvanizedaluminum sheet as automotive body sheet, the aluminum sheet preferablycomprises a weight per unit area of applied zinc in the range 5-100g/m², and more preferably in the range 5-40 g/m².

BEST MODES OF CARRYING OUT THE INVENTION

The invention will now be illustrated by several non-limitativeexamples.

EXAMPLE 1

Aluminum sheets manufactured from an AA3004 alloy suitable forapplication in building structures were pretreated and electrolyticallygalvanized in different ways in batch processes. The bonding of theapplied zinc layer was then tested by the so-called tape method. This isa rapid method for testing bonding. A piece of tape or self-adhesivetape, for example such as that used in offices, is stuck onto thegalvanized aluminum sheet and then pulled off again by hand. A valueassessment is then given to the bondability by allocating a numberwhere: (1)=excellent, (2)=good, (3)=fair, (4)=poor, (5) very poor. Forvalue assessments (1) and (2) the bonding was also tested using theknown zero bending test and the known lock-form test. For valueassessments (3) to (5) these supplementary bond tests were omitted. Thegalvanized aluminum sheets with value assessments of (1) or (2) werealso tested for durability in a corrosive, maritime industrialenvironment.

Table 1 gives the main process parameters used and the value assessmentfor the bond. Between the different steps of the pretreatment and thegalvanizing the aluminum sheets were rinsed with distilled water for atleast 5 s.

The degreasing in tests (1) to (14) was carried out using Percy 6340-29(trade name) produced by Henkel Metall Chemicals, concentration 10 g/l,treatment time 3 s, bath temperature approximately 65° C., directcurrent with a current density of approximately 10 A/dm².

The pickling in tests (1), (2), (9), (11) and (12) was carried out in anHCl environment (8% HCl solution) pH=1, treatment time approximately 5s. In tests (6) and (7) an 8% H₂ SO₄ solution was used and a treatmenttime of approximately 5 s.

The graining in tests (3) to (5) was carried out in 1% HCl solutionhaving a pH of about 1, bath temperature approximately 40° C.,alternating current of approximately 50 Hz, current density ofapproximately 50 A/dm² and variable treatment time.

The anodizing in tests (5), (7) to (9), (12) and (16) was carried outwith direct current, current density approximately 50 A/dm², polaritypositive, treatment time approximately 5 s, a ZnSO₄.H₂ O electrolytecomprising approximately 90 g/l zinc, pH=1.8, bath temperatureapproximately 50° C. In tests (10) and (11) the electrolyte comprisedsulphuric acid, other parameters were identical to test (5), except thatthe temperature was approximately 70° C. In tests (13) and (14)sulphuric acid and phosphoric acid respectively were used for theelectrolyte, pH=1.8, bath temperature approximately 60° C., stainlesssteel electrode material, alternating current, electrolysis timeapproximately 2 s, current density approximately 2 A/dm², totalimmersion time approximately 10 s.

In tests (15) to (17) the pretreatment was carried out by connecting thealuminum sheet A-C-A-C, electrolysis time per polarity approximately 3s, bath temperature approximately 20° C., current density approximately5 A/dm², immersion time approximately 10 s, composition of electrolyte100 g/l NaOH and 10 g/l ZnO. The thickness of the zinc layers formed bythis zincate treatment were in the range 0.1 to 0.5 g/m². In test (16)the aluminum sheet was anodized in addition to that treatment.

In tests (1) to (16) the aluminum sheets were electrolyticallygalvanized in a zinc sulphate electrolyte with 90 g/l zinc, pHapproximately 2, direct current with a current density of approximately50 A/dm², bath temperature approximately 50° C., immersion timeapproximately 20 s. In test (17) the pH was approximately 2.5, otherparameters being identical to tests (1) to (16). The electrolyticallyapplied zinc layer was approximately 35 g/m².

It may be inferred from the results given in Table 1 that a pretreatmentconsisting of degreasing and pickling (tests 1, 2 and 6) is insufficientto obtain a well-bonded layer comprising zinc.

A pretreatment consisting of degreasing and anodizing (tests 8, 10, 13and 14) is also insufficient to obtain a well-bonded layer comprisingzinc.

The combination of in succession degreasing, pickling and anodizing(tests 7, 9, 11 and 12) was also insufficient to obtain a well-bondedlayer comprising zinc.

Very good results were obtained when the aluminum sheet waselectrolytically grained following the degreasing (tests 3 to 5). Thebest results were achieved with the method in accordance with test (5),in which anodizing followed the graining.

Very good results were also obtained when the aluminum sheet waspretreated in an alkaline environment comprising zinc while beingA-C-A-C connected (tests 15 to 17). Good results were also obtained if,following on from that, an anodization step carried out (test 16).

The galvanized aluminum sheets obtained with the method in accordancewith tests (3), (5) and (15) to (17) were also subjected to the zerobending test and the lock-form test. In all cases the bond of the zinclayer and the aluminum sheet remained intact.

Galvanized aluminum sheets manufactured with the method in accordancewith tests (3), (5) and (15) to (17) were locally greatly deformed insuch a way that the galvanized aluminum sheets were comparable in shapeto KAL-ZIP and KAL-BAU. These galvanized aluminum sheets were thentested for durability by means of the Atmospheric Building CorrosionTest as described by B. Boelen in the article "New Product Test: TheAtmospheric Building Corrosion Test (ABC Test)", published on theoccasion of the ECCA Autumn Congress in Brussels on Nov. 27-28, 1995,and compared with untreated sheets of AA3004 alloy and pure zinc. After6 weeks in a maritime industrial surrounding there was no visibledifference between galvanized aluminum and pure zinc (Reinzink), whilethe AA3004 alloy was badly corroded particularly by the chloride. Amongother things it can be inferred from the results that galvanizedaluminum sheet manufactured in accordance with the invention has adurability comparable to a sheet of pure zinc.

                                      TABLE 1                                     __________________________________________________________________________    Degreasing                                                                             Pickling                                                                             Graining                                                                             Anodizing                                                 Temp                                                                             Time   Temp   Time   Time                                                                             Galvanizing                                                                         Bonding                                   Test                                                                             (° C.)                                                                    (s)                                                                              Solution                                                                          (° C.)                                                                    Solution                                                                          (s)                                                                              Solution                                                                          (s)                                                                              pH    rating                                    __________________________________________________________________________     1 65 3  HCl 50 --  -- --  -- 2     5                                          2 65 3  HCl 80 --  -- --  -- 2     5                                          3 65 3  --  -- HCl 30 --  -- 2     2                                          4 65 3  --  -- HCl  5 --  -- 2     3                                          5 65 3  --  -- HCl  3 ZnSO.sub.4                                                                        5  2     1                                          6 65 3  H.sub.2 SO.sub.4                                                                  70 --  -- --  -- 2     5                                          7 65 3  H.sub.2 SO.sub.4                                                                  70 --  -- ZnSO.sub.4                                                                        5  2     4                                          8 65 3  --  -- --  -- ZnSO.sub.4                                                                        5  2     4                                          9 65 3  HCl 50 --  -- ZnSO.sub.4                                                                        5  2     5                                         10 65 3  --  -- --  -- H.sub.2 SO.sub.4                                                                  5  2     5                                         11 65 3  HCl 50 --  -- H.sub.2 SO.sub.4                                                                  5  2     5                                         12 65 3  HCl 80 --  -- ZnSO.sub.4                                                                        5  2     5                                         13 65 3  --  -- --  -- H.sub.2 SO.sub.4                                                                  2  2     4                                         14 65 3  --  -- --  -- H.sub.3 PO.sub.4                                                                  2  2     4                                         15 Alkaline solution   --  -- 2     1                                         16 containing zinc +   ZnSO.sub.4                                                                        5  2     1                                         17 A-C-A-C connection  --  --   2.5 1                                         __________________________________________________________________________

EXAMPLE 2

The process as described in test 5 of Example 1 was further performed ineight test runs on a continuous pilot line using AA3004 sheet material0.26 m wide.

The process parameters used for the eight different test runs are listedin Table 2. In between graining and anodizing also the sheet was rinsedwith distilled water.

The galvanized sheet which was obtained was tested for the amount ofzinc layer, and the bonding was tested using the zero bending test.

Degreasing was performed in a tank with 10 g/l of Percy 6340-29 (tradename), with a direct current. After degreasing, a rinse step wasperformed. Electrolytic graining was carried out in 1% HCl solution,with alternating current of 50 Hz. Rinsing with distilled waterfollowed.

Anodizing was performed in a tank with an electrolyte having a pH ofabout 2 comprising 400 g/l ZnSO₄.H₂ O (resulting in about 90-100 gZn/l), and 30 g/l Al₂ SO₄.nH₂ O, and 30 g/l H₃ BO₃, and with a directcurrent. The H₂ BO₃ was added to act as a buffer.

The galvanizing was performed in a separate tank, but with the sameelectrolytic composition as used for anodising. A rinsing step followed.

From the results of Table 2 it can be seen that all the galvanized sheetmaterial had excellent bonding when tested in a zero bending test. Itcan also be seen that the line speed, and hence the treatment time, isan important process parameter for the amount of zinc coating on thealuminum sheet. At a line speed of 6 m/min (as in Run 5) the contacttimes for the various treatment steps were as follows: degreasing 3 s(repeated four times), graining 5 s, anodizing 5 s, and galvanizing 24s. The contact times at other line speeds can be calculated from thesevalues.

Depending on the local conditions a skilled person can therefore findvarious optima for the process parameters depending on the amount ofgalvanized zinc required on the aluminum sheet.

                                      TABLE 2                                     __________________________________________________________________________           Degrease    Graining                                                                              Anodizing                                                                             Galvanizing                                   Line    Current                                                                           Rinse   Current Current Current                                                                           Rinse                                                                             Zinc                              speed                                                                             Temp.                                                                             density                                                                           Temp.                                                                             Temp.                                                                             density                                                                           Temp                                                                              density                                                                           Temp.                                                                             density                                                                           Temp.                                                                             coating                                                                           Bonding                    Run                                                                              (m/min)                                                                           (° C.)                                                                     (A/dm.sup.2)                                                                      (° C.)                                                                     (° C.)                                                                     (A/dm.sup.2)                                                                      (° C.)                                                                     (A/dm.sup.2)                                                                      (° C.)                                                                     (A/dm.sup.2)                                                                      (° C.)                                                                     (g/m.sup.2)                                                                       rating                     __________________________________________________________________________    1  4   65  15  51  51  29  52   7.7                                                                              50    9.6                                                                             52   8.0                                                                              1                          2  12  65  12.8                                                                              46  53  29  52  15.3                                                                              51  32  52   5.5                                                                              1                          3  4   65  12.8                                                                              43  54  23  51  15.3                                                                              51  32  52  14.5                                                                              1                          4  2   65  12.8                                                                              41  57  23  51   7.7                                                                              51  32  52  30.5                                                                              1                          5  6   66  12.8                                                                              37  52  19  52  11.5                                                                              52  32  53  26.0                                                                              1                          6  4   66  12.8                                                                              41  48  19  51  11.5                                                                              54  32  53  25.5                                                                              1                          7  2   66  12.8                                                                              46  45  19  51   7.7                                                                              56  32  52  38.0                                                                              1                          8  4   66  12.8                                                                              49  45  38  52  14.2                                                                              58  32  52  41.0                                                                              1                          __________________________________________________________________________

What is claimed is:
 1. A method for applying a layer comprising zinconto at least one surface of an aluminum or aluminum alloy sheet,comprising the steps of pretreating said surface and applying the layerby electrolytic galvanizing, wherein the pretreating step compriseselectrochemical graining of said surface, in which the pretreating stepcomprises anodizing said surface after the electrochemical graining. 2.A method according to claim 1 in which the electrochemical graining isperformed in a solution having a pH less than 3 and in which analternating current is applied between the sheet and an electrode.
 3. Amethod according to claim 2 in which the electrochemical graining isperformed using a current density in the range 5 to 200 A/dm².
 4. Amethod according to claim 2 in which the alternating current frequencyis from 10 to 1000 Hz.
 5. A method according to claim 1 in which theduration of the electrochemical graining is from 0.2 to 60 s.
 6. Amethod according to claim 1 in which the galvanizing is performed usinga direct current having a current density of from 10 to 100 A/dm², abath temperature of from 20 to 70° C., a treatment time of from 5 to 90s, an electrolyte composition comprising zinc sulphate having an amountof zinc from 30 to 200 g/l, and an electrolyte pH of less than
 4. 7. Analuminum or aluminum alloy sheet having an electrolytically appliedlayer comprising zinc in which said layer is adjacent to an aluminum oraluminum alloy surface roughened by electrochemical graining inaccordance with claim
 1. 8. A method for applying a layer comprisingzinc onto at least one surface of an aluminum or aluminum alloy sheet,comprising the steps of pretreating said surface and applying said layerby electrolytic galvanizing, said pretreating step including applying apreliminary layer comprising zinc onto said surface, wherein theapplication of said preliminary layer comprises (i) immersing saidsurface in a zinc-containing alkaline solution, (ii) applying apotential to the sheet to cause an electrolytic current to flow with acurrent density during the application of said preliminary layer from0.5 to 20 A/dm², and (iii) reversing the polarity of said potential atleast once, in which said pretreating step includes anodizing saidsurface after applying said preliminary layer.
 9. A method according toclaim 8 in which the preliminary layer is applied in an amount of from0.1 to 0.5 g/m².
 10. A method according to claim 8 in which saidpolarity of said potential is reversed at least twice.
 11. A methodaccording to claim 10 in which the time interval between each twoadjacent polarity reversals is at least 0.5 s.
 12. A method according toclaim 8 in which during the application of said potential, said sheet isinitially an anode and is finally a cathode.
 13. A method according toclaim 8 in which said zinc-containing alkaline solution comprises from10 to 300 g/l of NaOH and from 2 to 40 g/l of ZnO.
 14. A methodaccording to claim 8 in which said electrolytic galvanizing is performedwith a direct current having a current density of from 10 to 100 A/dm2,a bath temperature of from 20 to 70° C., a treatment time of from 5 to90 s, an electrolyte composition comprising zinc sulphate in an amountof zinc from 30 to 200 g/l, and an electrolyte pH of less than
 4. 15. Analuminum or aluminum alloy sheet having an electrolytically appliedlayer comprising zinc, produced by a method in accordance with claim 8.16. A building sheet which is a roofing sheet comprising an aluminum oraluminum alloy sheet having an electrolytically applied layer comprisingzinc,produced by a method for applying the layer comprising zinc onto atleast one surface of the aluminum or aluminum alloy sheet, comprisingthe steps of: pretreating said surface and applying said layer byelectrolytic galvanizing, said pretreating step including applying apreliminary layer comprising zinc onto said surface, wherein theapplication of said preliminary layer comprises (i) immersing saidsurface in a zinc-containing alkaline solution, (ii) applying apotential to the sheet to cause an electrolytic current to flow with acurrent density during the application of said preliminary layer from0.5 to 20 A/dm², and (iii) reversing the polarity of said potential atleast once, in which the weight of said layer comprising zinc is from 10to 300 g/m².
 17. A building sheet according to claim 16 in which theweight of said layer comprising zinc is from 30 to 100 g/m².
 18. Amethod for applying a layer comprising zinc onto at least one surface ofan aluminum or aluminum alloy sheet, comprising the steps of:pretreatingsaid surface; and applying the layer by electrolytic galvanizing;wherein the pretreating step comprises electrochemical graining of saidsurface, subsequently followed by applying a preliminary layercomprising zinc onto said surface, wherein the application of saidpreliminary layer comprises (i) immersing said surface in azinc-containing alkaline solution, (ii) applying a potential to thesheet to cause an electrolytic current to flow with a current densityduring the application of said preliminary layer from 0.5 to 20 A/dm²,and (iii) reversing the polarity of said potential at least once,wherein said pretreating step includes anodizing said surface at a timeselected from the group consisting of after applying said preliminarylayer and after the electrochemical graining.
 19. An aluminum oraluminum alloy sheet having an electrolytically applied layer comprisingzinc produced by a method in accordance with claim
 18. 20. A buildingsheet comprising an aluminum or aluminum alloy sheet according to claim19 in which the weight of said layer comprising zinc is from 10 to 300g/m².
 21. An aluminum or aluminum alloy sheet having an electrolyticallyapplied layer comprising zinc produced by a method in accordance withclaim
 1. 22. A building sheet comprising an aluminum or aluminum alloysheet according to claim 21 in which the weight of said layer comprisingzinc is from 10 to 300 g/m².
 23. A building sheet, which is a roofingsheet, comprising an aluminum or aluminum alloy sheet having anelectrolytically applied layer comprising zinc in which said layer isadjacent to an aluminum or aluminum alloy surface roughened byelectrochemical graining,wherein the layer is applied by a method,comprising the steps of pretreating said surface and applying the layerby electrolytic galvanizing, wherein the pretreating step comprises theelectrochemical graining of said surface, in which the weight of saidlayer comprising zinc is from 10 to 300 g/m².